The present invention relates to a thermally developable, photosensitive material, and particularly to a thermally developable, photosensitive material which does not require the presence of mercury to suppress fogging.
A variety of methods which comprise subjecting photographic materials containing photosensitive components such as silver halide or the like to a so-called dry processing by heating to thereby obtain an image are known. Of these photosensitive materials which can form photographic images using dry processing, the most common one is a thermally developable, photosensitive material as described in U.S. Pat. Nos. 3,152,904, 3,457,075, 3,707,377 and 3,909,271, in which an oxidation-reduction image forming composition comprising, as essential components, organic silver salt oxidizing agents (for example, silver behenate), photocatalysts such as photosensitive silver halide, and reducing agents (for example, 2,2'-methylenebis [4-methyl-6-t-butyl] phenol), is utilized. While the thermally developable, photosensitive material is stable at ambient temperature, after exposure to light, the organic silver salt oxidizing agent and reducing agent present in the photosensitive layer undergo, when heated generally at temperatures of higher than about 80.degree. C., preferably greater than about 100.degree. C., an oxidation-reduction reaction due to the catalytic action of the photocatalyst which is present in proximity to the organic silver salt oxidizing agent and reducing agent to thereby form silver. The exposed areas of the photosensitive layer are rapidly darkened so that a contrast is formed between the unexposed areas (background) to form an image.
Mercury, in the form of mercuric salts such as mercuric bromide or mercuric acetate, is generally included in the thermally developable, photosensitive material to suppress background darkening or fog upon processing. Birkeland, U.S. Pat. No. 3,589,903 discloses that by incorporating mercuric ion in a light-sensitive heat-developable imaging sheet containing catalytic amounts of light-sensitive silver halide in catalytic association with organic silver salt oxidation-reduction image-forming means, the sheet is given increased speed, stability and contrast. Ulbing, U.S. Pat. No. 3,692,526 discloses that a combination of a mercury salt, such as a mercury halide with thiourea dioxide in a heat-processable photosensitive element, composition and/or process provides reduced background print-out. When the mercury salt, e.g., mercuric chloride, or thioruea dioxide is used alone in the absence of the combination, no satisfactory image is produced.
It is well known that excessive exposure to mercury can result in corrosive effects on skin and mucous membranes, nausea, vomiting, abdominal pain, and kidney damage. Elimination of at least a portion of the mercury from paper and film would be beneficial to those involved in the manufacturing process and to the environment after disposal. Accordingly, it would be highly desirable to provide an additive to photothermographic elements which would prevent fog formation but not be toxic to humans or to the environment.